Layer arrangement provided with a structure producing a diffractive optical effect and a lens-type effect

ABSTRACT

A layer arrangement is proposed, particularly for transfer films or laminated films, which exhibits at least two superposed synthetic resin layers, between which there is provided an interface surface having a refractive structure producing a lens-like effect, the novelty claimed being a special design of the structure having a diffractive effect.

The invention relates to a layer arrangement, especially forimplementation in transfer films or laminated films, which exhibits atleast two superposed material layers, of which at least that (or those)facing the observer in use has or have a transparent or semi-transparentappearance, and between which an interface is formed which exhibits, inat least one area thereof, a diffractive optical structure producingsome lens-like effect, either magnifying or de-magnifying.

In this context, transfer films include especially so-called embossingfilms, which consist of a base film and a transfer layer that isdetachable from the base film for transference to a substrate. Usuallythe transfer layer of embossing films is composed of lacquer layers,which means that, in the present invention, the term “material layer”principally means lacquer layer, and at times also adhesive layer.However, the invention also encompasses other embodiments, in which a“material layer” is formed by ambient air or a metallic, dielectric, orsemiconductor coating. The structure of laminated films coincidessubstantially with that of transfer films, with the exception howeverthat the synthetic resin layers or lacquer layers are not detachablefrom the base film, but rather can be affixed together with the basefilm to a substrate. Transfer films and laminated films with layerarrangements of this sort are in particular used for security purposes,although they are also used in decorative applications.

Layer arrangements of the type mentioned above are currently known andare coming into use, for example, in the form of a lens having a uniformappearance and used as security device in credit cards (Amex-Blue) newon the market. In these credit cards, the lens-like effect is manifestedover an area of comparatively large diameter, and has substantially theform of a circular lens. In the lens-like effect produced by diffractiveoptical structures of known layer arrangements, a structure produced bya holographic technique is used, which in general possesses a sinusoidalsurface profile. Such holographically manufactured lenses have manyshortcomings, quite apart from the fact that the holographic manufactureof diffractive optical structures with lens effects, with comparablysmall technical effort, is only possible when lenses having circular or,at best, elliptical shapes are involved. One drawback of holographicallyproduced lenses is, for example, that they are not very bright inappearance and in general they exhibit irregularities, especially in thecentral area, whereby the visual effect that the lens should produce isconsiderably degraded. A further disadvantage of holographicallyproduced lenses is that it is virtually impossible to achieve certaincolor effects with any great freedom of design.

It is an object of the invention to provide an arrangement of layers ofthe above mentioned type which does not have the described disadvantagesof the known, holographically-produced lens structures having sinusoidalsurface profiles, ie to design the structures giving the lens-likeeffect in a way that allows them to be produced in reasonable time withworkable technology, in very precise and varied forms, causes theefficiency and brightness of the effects due to the lens-like structureto be much improved in comparison with the effects produced byholographically produced structures, and, finally, provides at leastconsiderably greater freedom in the production of color effects than ispossible with holographically produced structures.

This object is achieved, according to the invention, by the proposalthat the diffractive optical structure producing the lens-like effect(hereinafter referred to as “lens structure”), be designed such that thegrating structure, including the line frequency and, as necessary, othergrating constants, be varied continuously over the surface of thestructure to form a binary structure or some similar structure in whichone of the walls of each grating groove run parallel to each other andapproximately parallel to a perpendicular to the principal plane of theinterface layer, while, at least as an average value taken over theentire groove wall, the angle of the other wall of each grating grooverelative to a perpendicular to the principal plane of the interfacelayer varies substantially continuously, the grating depth (9) being notgreater than 10 μm.

“Binary structure” in the present description is understood to mean astructure in which the grating grooves and the grating bars havesubstantially rectangular cross sections, whilst for the production oflens effects the grating constants will have to be continuously variedfrom the center of the lens to its edge, however, which in general meansthat both the groove width and the bar width will vary in binarygratings. Sufficiently fine binary gratings are easily produced with theuse of appropriate masks, which results not only in much greasteraccuracy, but also in comparatively lower manufacturing costs.

The other claimed embodiment of the grating structures will preferablybe produced by means of the so-called “direct-writing” process,employing laser beam or electron beam lithographic printers. When usingthese methods, it is easy to produce very precise grating structures,especially the structure claimed herein, in which one wall of each ofthe grating grooves runs approximately perpendicular to the principalplane of the lens-forming grating, while the other wall is at a slantcausing tapering of the grating groove toward the grating base. In thisconnection it is also possible to form the oblique walls not with acontinuous profile, but rather to approximate a stepwise arrangement,and for many applications partitioning in four or eight steps willsuffice. However, it is also possible, where high quality is required,to provide, say, 64 steps.

Regarding the design of such gratings reference is made, for the sake ofsimplicity, to FIG. 1, in which drawing a) shows the cross section of anormal, refractive lens, while the middle drawing b) showsdiagrammatically the cross section of a diffractive lens with one wallof each grating groove running perpendicular to the principal plane ofthe grating while the opposing wall runs obliquely. In drawing c) ofFIG. 1, a so-called “binary structure” is shown, in which the gratinggrooves and the grating bars both exhibit rectangular cross sectionsand, as can be seen in FIG. 1 c), the width of the grating bars and thewidth of the of the grating grooves decrease from the center of the lensto its edge. All three of the lens forms shown in FIG. 1 producefundamentally the same optical effect for any particular wavelength.

However, what is special about the invention's proposed diffractive lensstructures is that, unlike refractive lenses, they create differentvisual impressions depending on the wavelengths of light that arepresent. Nevertheless, the height of diffractive lenses patterned afterthe designs shown in FIG. 1 b) and FIG. 1 c) is many times smaller thanthe thickness of the corresponding refractive lens illustrated in FIG. 1a). Through this method, it is for the first time possible to integratethe lens structure into a layer arrangement without having to work withextreme layer thicknesses, which must be regarded as being impossiblefor all practical purposes.

When lens structures according to the invention are used, the firstadvantage obtained is that higher efficiency than that achievable byholographically manufactured lenses can be achieved, which consequentlymeans that the picture, decorative effect, or security effect made withthe aid of the lens will be brighter. Another advantage is that the lensstructures according to the invention can be produced with very greataccuracy in comparison with holographically produced structures—wherebythe visual appearance is significantly improved. A final advantage isthat by suitably selecting the grating constants (line frequency, groovedepth, etc) it is possible, with the structures of the invention, toachieve special color effects, or to control the color effects in apredetermined way over the overall profile of the lens structure.Furthermore, in this connection one should mention the possibility ofcombining lens structures with other elements that produce opticaleffects, eg other types of diffractive structures for achieving motioneffects, flips, or similar effects, or with thin-film structures forproducing special color effects, as is generally known from, say,optically variable security devices. The lens structures of theinvention thus have, in comparison with holographically producedstructures, besides the commonality of their small “thickness”, a largenumber of advantages.

Layer arrangements having lens structures according to the invention canproduce the pertinent special optical effects for observation intransmission as well as in reflection. To make viewing in transmittedlight possible, the invention proposes that the layers adjacent theinterface layer be transparent and show a distinct difference in theirrefractive indices of, preferably, at least 0.2. The difference inrefractive index causes the lens action of the interface to produce adistinctly visible optical effect, in spite of the fact that the lightpasses through the layer arrangement. A special feature of working intransmission is that the grating need not be covered on one side, butcan instead be exposed to air.

It is further within the scope of the invention that the interface, atleast over some of its area, has a reflectivity-enhancing layer, anexpedient reflectivity-enhancing layer being a metallic layer, forexample one produced by vapor-deposition. However, it is by all meansconceivable to consider a transparent reflectivity-enhancing layerhaving an appropriately high refractive index, in which case, the layerarrangement could be made transparent to a certain degree. Thin-filmarrangements of known layer combinations or semiconductor layers couldalso be used.

The holographically produced security device in known credit cards,which is made using conventional layer arrangements, contains only asingle, circular lens structure. On the other hand, using a diffractivelens structure of the invention, it is possible to place a plurality oflens structures over the surface of the layer arrangement, by whichmeans much more interesting effects can be achieved (for use indecorative applications) or, where the lens structure is part of asecurity device, an enhanced security effect can be attained.Advantageously, in the latter case, the multiple lenses can be arrangedgridwise, so that verification can be made easier. Alternatively, atleast partially overlapping areas of the lens structures areconceivable, in which case even nesting might be achieved such thatdifferent lens structures would appear at different angles ofobservation.

The manufacture of such lens structures or lens structure arrangementswill be particularly easy if, as proposed by the invention, the lensstructures are substantially circular, taking the form of concentricgrating lines.

In practice, it has proven to be expedient if the diameter of the lensstructures lies between 0.15 and 300 mm, and preferably between 3 and 50mm.

If, as also provided by the invention, the grating depth of the lensstructure is less than 5 μm, and preferably less than 3 μm, such gratingstructures can be readily incorporated into the lacquer layers, whichhave approximately this thickness, of transfer films or laminated films.

According to the invention, it is proposed that the binary structurehave approximately the same depth over the entire surface of the lensstructure. This facilitates manufacture greatly. The choice of the depthof the binary structure influences the color perceived by the observerlooking at the lens structure.

Finally, it can be advantageous if the transparent layer (or layers)seen by observer is (are) colored without the use of pigment.

Other characteristics, details, and advantages of the invention will beapparent from the following description of examples of preferredembodiments with reference to the drawings, in which:

FIG. 1 shows diagrammatically and in cross section

-   -   a) a refractive lens.    -   b) a diffractive lens having grating grooves of approximately        triangular cross sections, and    -   c) a lens with a diffractive binary structure;

FIG. 2 is a diagrammatic top view of a security device or decorativeelement with a layer arrangement of the invention and having a lensstructure of the invention; and

FIG. 3 is a representation similar to FIG. 2 but on a smaller scale,showing a grid-wise arrangement of a plurality of lens structures.

In the diagrammatic cross sectional views of FIG. 1 it is shown that thelayer arrangement in accordance with the invention comprises twomaterial layers 1 and 2, which form an interface layer 3 between them,which can be metallized for example, this being achieved by, say, vacuummetal vapor deposition. For certain applications the material layers 1and 2 can be formed by air. The diameter of the lenses in FIG. 1 isspecified along the x-axis in arbitrary units, as the exact size ordiameter of the lens structure is not relevant. However, in general, thediameter of the lens structures lies between 0.5 and 300 mm, preferablybetween 3 and 50 mm, the focal length being usually between the value ofthe lens diameter and five times this value.

On the y-axis in FIG. 1, the thickness of the material layers 1, 2 orthe height of the structure is given, with the values representing thephase difference in radians. By using a particular wavelength of light(eg 550 nm for the maximum sensitivity of the human eye) one cancalculate the geometric depth from this phase difference in known manner(including accounting for the corresponding refractive indices). From acomparison of FIG. 1 a) with FIGS. 1 b) and 1 c), it is clear that thethickness of the layer arrangement represented in 1 a) must be at leastten times greater than the thickness of the layer arrangementrepresented in 1 b) and approximately twenty times greater than thethickness of the layer arrangement of FIG. 1 c). That the layerarrangements of FIGS. 1 b) and 1 c) can be substantially thinner thanthat of FIG. 1 a) has to do with the small overall height 9 of the lensstructure due to the interface layer 3, which covers a height that,calculated for FIG. 1 b) (for a system n=1.5/n=1 in transmission), isonly approximately twice the wavelength, and calculated for FIG. 1 c),is approximately equal to the wavelength.

Layers 1 and 2 of the layer arrangement are in general lacquer layers ofappropriate composition, with at least the layer facing the observer (inthe present cases usually layer 1) being substantially transparent,although it can be colored, if desired. For certain applications, one ofthe layers can be an adhesive layer and the layer facing the observercan be omitted.

If the interface layer 3 is metallized or provided with some otherhighly reflective coating, layer 2 can likewise be transparent oralternatively translucent or opaque. If, on the other hand, the layerarrangement according to the invention is used in transmission, forexample as a cover of an existing visible characteristic on a substrate,layer 2 must also be transparent. In this case interface layer 3 wouldnot have a metal coating, which is generally opaque. Instead, the twotransparent layers 1 and 2 would be chosen such that their refractiveindices differ (the difference in refractive index being preferably atleast 0.2), so that, despite the use of two transparent layers, theeffect produced by the interface layer 3 will be visible with adequateoptical clarity.

The lens structure represented in 1 b) is usually produced in a “directwriting process”, ie in a process in which either, using a laser, thematerial is shaped by ablation to make it conform with the desiredprofile, or, using a laser or an electron beam lithographic printer, aphotoresist patterned according to the desired profile is exposed andthen the desired profile or its negative is obtained by developing thephotoresist. This procedure offers the advantage that it can producevery different grating structures and, especially, very differentgrating cross sections, eg for certain applications so-called blazedgratings. Particularly noteworthy is the fact that the angle α formedbetween the oblique grating groove walls 4 and a perpendicular S to theprincipal plane of the lens structure can, as is clearly visible in FIG.1 b), vary continuously from the lens center to the edge, especiallyconsidering the fact that the grating groove walls 5 that run parallelto the perpendicular S form a quasi-discontinuity in an otherwisesubstantially smooth lens profile, formed by the other oblique gratinggroove walls 4, as well as the central parabolic section 6 of interface3.

Such lens structures, as well as the way to compute them, are basicallydescribed in the literature, and so will not be treated further here.

Mention may be made of the possibility of using, instead of a continuousslant of walls 4 over their height 9, as shown in FIG. 1 b), astep-shaped arrangement, in which the surfaces forming the stepsapproach the optical effect provided by slanting walls 4. Such gratingstructures can be produced either by use of the so-called direct-writingprocess or by using appropriate masking techniques, the number of stepsbeing varied depending on the desired results. For many applications, apartition in four or eight steps is sufficient. Where higher quality isrequired, it is also possible to provide, say, sixty-four steps, or anumber equal to a higher power of 2.

The binary structure represented in FIG. 1 c) is produced by the use ofappropriate masks. The essential characteristic of the binary structure,as shown in FIG. 1 c), lies in the fact that both the grating grooves 7and the grating bars 8 are essentially rectangular in cross section.Another special characteristic of the structure shown in FIG. 1 c) isthat the grating depth 9 is uniform over the entire lens structure,which offers the advantage, especially for fabrication, that neither isit necessary to employ different activation times for thematerial-removing medium nor is it necessary to work with differentintensities of the medium passing through the mask to act on thesubstrate.

FIG. 2 is a diagrammatic drawing (in reality the spacing of the gratinglines is much smaller) showing a lens-like element that is produced witha lens structure like that shown in FIG. 1 b), with the top view of FIG.2 clearly showing the steadily decreasing separation between theindividual grating bars and the steadily increasing groove frequencyfrom the center of the circular lens out to its edge. In addition, onecan see how the inclination of the groove walls 4, which are visible inthe plan view of FIG. 2, changes steadily and in a substantiallycontinuous fashion, from the center of the lens outwards. The groovewalls 5, which are perpendicular to the principal plane of the lens, areclearly visible in FIG. 2 as dark lines.

FIG. 3 shows a further possibility of how diffractive lens structuresmight be designed in a layer arrangement according to the invention.

In the application example shown in FIG. 3, which could, for example, berealized in a decorative transfer film or laminated film, circular lensstructures, that in principle could have the lens structure of FIG. 2,are distributed over the surface of the film in a number of regions,which form a grid pattern. The arrangement is configured such that theouter grating grooves are not truncated, as is the case with some of theouter grooves shown in FIG. 2 The lens structures 10 of FIG. 3 are, onthe contrary, all substantially circular. The spheroid-square spacescreated between the circular lens structures by their adjacent placementare filled, in the layer arrangement of FIG. 3, with appropriatelyshaped diffractive structures 11, which can, if desired, also produce alens effect, the lens structures 10 having for example the effect ofconverging lenses, while the structures 11 act as diverging lenses, bywhich means the optical effects of both lens types are quasienhanced.

It is obviously possible, by appropriately combining different lensstructures, to produce layer arrangements showing complex opticaleffects, while it is naturally also possible to design other, locallydefined, diffractive structures, that generate completely differentkinds of effect, for example motion effects, flips, etc. It is alsoconceivable to combine the lens structures and/or other diffractivestructures with a series of thin films of special colors, eg OVI, orwith semiconductor layers, in order to achieve special color-changingeffects.

Particularly interesting embodiments of the layer arrangement areproduced when the interface layer 3 is only partially metallized. Forexample demetallization in register with the lens structures could becarried out.

Furthermore the lens structures obviously do not always have to be of acircular shape like those generally depicted in the drawings. Aparticular advantage gained by using diffractive lens structures is thatthey can be superposed over other forms (so-called free-form surfaces),in order to obtain, for example, configurations having athree-dimensional appearance. Furthermore it would also be conceivable,for example, to divide the lens structures of FIG. 2 into parts and toput these parts together in a different way, again obtaining veryinteresting optical effects.

1. A layer arrangement, particularly for transfer films or laminatedfilms, which exhibits at least two superposed layers of material, ofwhich at least that (or those) facing the observer in use is or aretransparent and between which an interface is formed which exhibits, atleast in one area thereof, a diffractive optical structure producingsome lens-like effect, either magnifying or de-magnifying, wherein thediffractive optical structure producing the lens-like effect (the “lensstructure”) is designed such that the grating structure, including theline frequency and, as necessary, other grating constants, is variedcontinuously over the surface of the structure to form a binarystructure or some similar structure in which one of the walls of eachgrating groove run parallel to each other and approximately parallel toa perpendicular to the principle plane of the interface layer, while theangle of the other wall of each grating groove relative to aperpendicular to the principle plane of the interface layer variessubstantially continuously over the area of the lens structure, thegrating depth of the lens structures being not more than 10 μm.
 2. Alayer arrangement as defined in claim 1, wherein the layers adjacent theinterface surface are transparent and exhibit a different refractionindex, preferably one differing by at least 0.2.
 3. A layer arrangementas defined in claim 1, wherein the interface surface is provided, atleast in certain regions, with a reflectivity-enhancing layer.
 4. Alayer arrangement as defined in claim 3, wherein thereflectivity-enhancing layer is a metal layer.
 5. A layer arrangement asdefined in claim 1, wherein any one of the a number of lens structuresare distributed over the area of the layer arrangement.
 6. A layerarrangement as defined in claim 5, wherein said multiple lens structuresare arranged grid-wise.
 7. A layer arrangement as defined in claim 1,wherein the lens structures are substantially circular and haveconcentric grid lines.
 8. A layer arrangement as defined in claim 1,wherein the lens structures have a diameter ranging from 0.15 to 300 mm,preferably from 3 to 50 mm.
 9. A layer arrangement as defined in claim1, wherein the grating depth of the lens structures is less than 5 μmand preferably less than 2 μm.
 10. A layer arrangement as defined inclaim 1, wherein the binary structure has approximately the same depthover the entire area of the lens structure.
 11. A layer arrangement asdefined in claim 1, wherein the transparent layer(s) facing the observerare colored without the use of pigments.